The 45-year-old actress reveals her cancer has spread in an exclusive sit-down with ET.

"I had breast cancer that spread to the lymph nodes, and from one of my surgeries we discovered that some of the cancer cells might have actually gone out of the lymph nodes," Doherty tells ET's Jennifer Peros. "So for that reason, we are doing chemo, and then after chemo, I'll do radiation."

Doherty also reveals she had a single mastectomy in May and is open about the most difficult part of fighting cancer.

"The unknown is always the scariest part," she shares. "Is the chemo going to work? Is the radiation going to work? You know, am I going to have to go through this again, or am I going to get secondary cancer? Everything else is manageable. Pain is manageable, you know living without a breast is manageable, it's the worry of your future and how your future is going to affect the people that you love."

Doherty through the years:

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Shannen Doherty through the years

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Shannen Doherty through the years

WEST HOLLYWOOD, CA - APRIL 19: Actress Shannen Doherty attends Jennie Garth's 40th birthday celebration and premiere party for 'Jennie Garth: A Little Bit Country' at The London Hotel on April 19, 2012 in West Hollywood, California. (Photo by Angela Weiss/Getty Images)

LOS ANGELES, CA - DECEMBER 14: Shannen Doherty signs copies of her new book 'Badass' at Barnes & Noble bookstore at The Grove on December 14, 2010 in Los Angeles, California. (Photo by Jason LaVeris/FilmMagic)

Shannen Doherty arrives at the Los Angeles premiere of 'Burning Palms' held at ArcLight Cinemas on January 12, 2011 in Hollywood, California.

NEW YORK - NOVEMBER 02: Actress Shannen Doherty promotes 'Badass' at Borders Books & Music, Columbus Circle on November 2, 2010 in New York City. (Photo by Jason Kempin/Getty Images)

Shannen Doherty during DKNY Jeans Presents Vanity Fair in Concert With a Live Performance by Alanis Morissette Benefiting Step Up Women's Network at Hiro Ballroom at The Maritime Hotel in New York City, New York, United States. (Photo by Jamie McCarthy/WireImage)

Shannen Doherty during Olympus Fashion Week Fall 2005 - Sweetface by Jennifer Lopez and Andy Hilfiger - Backstage Arrivals at The Tent, Bryant Park in New York City, New York, United States. (Photo by KMazur/WireImage)

But Doherty has found amazing support from her team of doctors, including the surgeon who performed her mastectomy.

"He didn't want me to wake up with absolutely nothing," she recalls. "It was very important to him that I didn't have that experience, and so, he put an expander in, so I have a tiny something there. It's cute and sometimes we fill it up and make it bigger, and sometimes we reduce it."

"I'm being so personal right now," she acknowledges. "But I think a lot of women can probably go, 'Yeah, I've been through that.' It's great though, you get to pick out what size you want."

Still, Doherty did have an emotional reaction to getting fitted for a new bra.

"It was traumatic and horrible, and I didn't think anything of it at the time, then my mom went with me and I broke down crying in the dressing room and ran out," she recalls, now able to laugh about it. "And then sat in the car crying."

Doherty was diagnosed with breast cancer in February 2015, and has undergone three out of eight rounds of chemotherapy so far. One of the more difficult experiences has been losing her iconic hair -- the long dark locks and blunt bangs she has sported since her Brenda Walsh days.

Thankfully the Beverly Hills, 90210 actress has the support of her mom, best friends, and her husband of five years, Kurt Iswarienko. With their help she was able to turn a heartbreaking moment into an inspiring one when she shared photos of herself shaving her head earlier this month.

A photo posted by ShannenDoherty (@theshando) on Jul 19, 2016 at 9:05pm PDT

"After my second treatment, my hair was really matted, like in dreadlocks. And I went to try and brush it out, and it just fell out," Doherty recalls. "I just remember holding onto huge clumps of my hair in my hands, and just running to my mom crying, like, 'My hair, my hair, my hair, my hair.'"

"It was just shedding and it was driving me crazy," she continues. "It was just clumps, and I was like, 'Just grab the kitchen scissors.' And my mom's like, 'Wait, wait, wait.' I'm just, like, 'Grab it.' She went and grabbed the kitchen scissors, and put it in a ponytail and she just chopped it off. And it was this cute little bob, but it wasn't enough, you know, it was falling out."

A photo posted by ShannenDoherty (@theshando) on Jul 19, 2016 at 9:06pm PDT

Unfortunately, her husband wasn't able to be there with her during the process, but he was still cheering her on.

"He was in Mexico working and he was texting nonstop in the middle of his photoshoot like, 'Give me pictures, and are you ok? I wish I could be there,'" she shares. "He was so stricken that he wasn't there with me but I sent him pictures and videos and he was like, 'Oh my god. You're hot with no hair.' So you know, everybody kind of made me feel better about it."

Doherty, who wears scarves over her head -- some personally sent to her by designer Diane von Furstenberg -- also candidly describes her intense battle with the disease.

"I am in bed and it is a rush to the bathroom," she says about her after-chemo experience. "It's, you know, you're throwing up every single second. After my first treatment I lost 10 pounds, instantly. You're throwing up and the last thing you want to do is be in a car. You don't want to be moved, you can't eat. But my husband has to, you know, pick me up. He puts me in the car, he buckles me in, and he drives me to my oncologist and they hook me up to an IV and I get hydrated. ... There's nothing going in my body, it's all going out."

Doherty admits to being self-conscious about her appearance. Though, above all, she hopes to inspire others fighting cancer.

"People don't realize that cancer -- yes, it ages you -- but also, you can balloon up from the various meds," she explains. "There are so many different reactions you have and so I just -- I didn't want someone to take a picture of me coming out of the grocery store and be like, 'Oh God, look at her.'"

"I wanted to put it out there the way it felt the best for me to put it out there," she adds. "And also, if I could help one person then it makes me go, 'Oh OK.' It's easier to live with having cancer if I know I'm helping at least one person."